The immediate symptom suggests the SSH daemon is terminating the connection during the initial protocol exchange. The debug output showing missing host keys (/etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key) points to a critical configuration problem.

First, check if the SSH service is actually running:

sudo systemctl status sshd
# OR for older systems
service ssh status

If the service isn't running, start it with:

sudo systemctl start sshd
sudo systemctl enable sshd

The standard key generation commands might not properly set permissions. Try this comprehensive approach:

# Remove existing keys (backup first if needed)
sudo rm /etc/ssh/ssh_host_*
# Regenerate all key types with proper permissions
sudo ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key -N ""
sudo ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""
# Set strict permissions
sudo chmod 600 /etc/ssh/ssh_host_*
sudo chmod 644 /etc/ssh/ssh_host_*.pub
# Restart SSH service
sudo systemctl restart sshd

Since you're using a non-standard port (7070), ensure proper configuration:

# Check sshd_config port setting
grep ^Port /etc/ssh/sshd_config
# Verify firewall rules
sudo ufw status
sudo iptables -L -n

Examine these critical SSH configuration parameters:

# Check for any restrictive settings
sudo grep -E '^(Allow|Deny)(Users|Groups)' /etc/ssh/sshd_config
# Verify authentication methods
sudo grep -E '^(PasswordAuthentication|PubkeyAuthentication)' /etc/ssh/sshd_config

Run SSH in debug mode on the server to get more detailed information:

sudo /usr/sbin/sshd -d -p 7070

This will keep the daemon in foreground mode with debug output, showing exactly where the connection fails.

Test basic connectivity from another machine:

nc -zv your.server.ip 7070
# If netcat isn't available, use:
telnet your.server.ip 7070

Always check the system logs for additional clues:

sudo tail -f /var/log/auth.log
# Or on newer systems:
sudo journalctl -u sshd -f

If all else fails, perform a complete reinstallation:

sudo apt purge openssh-server
sudo rm -rf /etc/ssh
sudo apt install openssh-server
sudo ssh-keygen -A
sudo systemctl restart sshd

---

The error message indicates that the SSH handshake process is failing during the initial key exchange phase. From your debug output, two critical host keys appear to be missing: ssh_host_rsa_key and ssh_host_dsa_key.

First, let's properly regenerate the missing host keys with correct permissions:

sudo rm /etc/ssh/ssh_host_*
sudo ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key -N ""
sudo ssh-keygen -t ecdsa -f /etc/ssh/ssh_host_ecdsa_key -N ""
sudo ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""
sudo chmod 600 /etc/ssh/ssh_host_*
sudo chmod 644 /etc/ssh/ssh_host_*.pub

Check your /etc/ssh/sshd_config for these essential settings:

Port 7070
Protocol 2
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
LoginGraceTime 120
PermitRootLogin prohibit-password
StrictModes yes
MaxAuthTries 6
MaxSessions 10

After making changes, always verify your configuration:

sudo sshd -t
sudo systemctl restart ssh
sudo systemctl status ssh

Since you're using a non-standard port (7070), ensure:

sudo ufw allow 7070/tcp
sudo netstat -tulnp | grep 7070

For deeper analysis, run sshd in debug mode:

sudo /usr/sbin/sshd -d -p 7070

Then attempt connection from another terminal with maximum verbosity:

ssh -vvv -p 7070 user@host

If standard SSH fails, try these diagnostic approaches:

# Test basic TCP connectivity
nc -zv host 7070

# Check for SELinux issues
sudo ausearch -m avc -ts recent

# Verify PAM authentication
sudo tail -f /var/log/auth.log

These commands help identify system-level issues:

# Check for disk space issues
df -h

# Verify system logs
journalctl -u ssh --no-pager -n 50

# Check for resource constraints
ulimit -a
free -h